The present invention relates to a heat exchanger assembled with zinc deposited thereon by using a flux containing zinc halide and zinc powder, and a method of producing the same.
Aluminum and aluminum alloys hereinafter referred to as "aluminum materials" are widely used for heat exchangers for automobiles, aircrafts and ships and other machines and facilities, because of their lightness, workability, corrosion resistance and low expense.
The assembling of heat exchangers using aluminum materials is carried out mainly by using Al-Si filler in a vacuum without using fluxes.
On the other hand, the assembly of heat exchangers is also known to be carried out by soldering using a flux containing zinc chloride in the atmosphere to utilize zinc deposited therefrom as solder. This reaction soldering method is advantageous in that:
(1) It is not necessary to use a brazing sheet clad with filler.
(2) Since the temperature in reaction soldering is as low as about 350-500.degree. C., the aluminum material is not deteriorated.
(3) Since the aluminum material is covered with zinc deposited on the surface thereof after the reaction soldering, high corrosion resistance is achieved That is, since Zn used for soldering is electrochemically less nobel than Al, it serves to improve the corrosion resistance of a tube when used in practical applications.
Because of the above advantages, the reaction soldering method of aluminum materials using a flux containing zinc chloride in the atmosphere has been used in part for assembling heat exchangers.
However, this reaction soldering method has its own problems:
(1) Since a large volume of flux containing a high concentration of zinc chloride is used, a large amount of AlCl.sub.3 gas is generated during the reaction soldering, extremely deteriorating the working environment.
(2) A residue such as a remaining flux should be removed after the reaction soldering, but this residue can be removed only with water washing, so that the removal of the residue is difficult.
(3) Since the flux may not be fully reacted or may not spread evenly, zinc is not deposited evenly on the surface of the aluminum material, so that it does not have a good surface condition, and the improvement of corrosion resistance cannot be expected all over the surface of the aluminum material.
As a matter of fact, the above-described reaction soldering method using a flux containing zinc chloride was proposed by Clyde S. Simpelaar et al. in 1965 (U.S. Pat. 3301688) and was used in the past. This reaction soldering method, however, suffers from the generation of a large amount of AlCl.sub.3 gas, so it has been fading out since about 1970.
That is, because of the generation of AlCl.sub.3 gas, it may be said that the reaction soldering method using a flux containing zinc chloride is not used at present. Instead of this reaction soldering method, a vacuum brazing method and an inert gas brazing method which do not suffer from the problem of AlCl.sub.3 gas generation are now being used.
The vacuum brazing method proposed by General Electric between 1967 and 1969 (U.S. Pat. No. 3321828, 3322517, 3373482,3328914 and 3400918) and the inert atmosphere brazing method proposed by V.A.W during 1971-1972 (West German Laid-Open Patent Applications 1962760 (1971), 2129460 (1972)) have received much attention as means for providing large amounts of brazed products with little pollution, developing wider applications.
These brazing methods, however, use Al-Si filler, and the brazing temperature is as high as 600.degree. C. Therefore, products being brazed may partially buckle during the brazing operation, and the brazed products may have decreased mechanical strength. Further, in the vacuum brazing method, it is known that Zn contained in the material for enhancing its corrosion resistance is evaporated during the brazing. Accordingly, the reaction soldering method, which can be carried out at a temperature as low as 400-450.degree. C. without such problems, has been being used, though not widely, for special applications, despite the fact that it suffers from an environmental problem.